In wireless communications networks, antenna design may be key to obtaining good performance and capacity. This applies for the wireless communication from a network node to a wireless user terminal or conversely, as well as between two network nodes. For example, massive beam forming, i.e., beam forming using active antenna arrays where antenna elements are orders of magnitude more numerous than in current communications networks, is expected to become a technical component in the radio access part of future fifth generation (5G) communications networks. By the deployment of large antenna arrays at the radio base stations, user data can be transmitted focused in space so that energy is received mainly by the wireless device dedicated by the user data, thus resulting in little interference being perceived by other wireless devices or other types of nodes. Massive beam forming therefore has the potential to increase system capacity and energy efficiency considerably.
According to the Long Term Evolution (LTE) standard, codebooks may be constructed using Discrete Fourier Transform (DFT) based vectors, enabling beamforming towards a fixed set of angular directions. A DFT beam, by its nature, is characterized with a fixed power pattern, whose half-power beamwidth (HPBW) is determined by the parameters of the antenna array (number of antennas, antenna element spacing, HPBW of the power pattern of an antenna element). For example, an antenna array having eight equidistant omnidirectional antennas with a nearest-neighbor spacing of 0.8 wavelengths, excited with a DFT vector will yield a power pattern with a HPBW of 8 degrees.
Generally, the power pattern of a DFT-beam pointing at the angle ϕ0 transmitted from a ULA having N antennas can be described by the Dirichlet kernel |G(θ)| as follows:
                G      ⁡              (        θ        )                  =                                    sin          ⁡                      [                          N              ⁢                                                          ⁢              π              ⁢                                                          ⁢                                                d                  λ                                ⁡                                  (                                                            sin                      ⁢                                                                                          ⁢                      ϕ                                        -                                          sin                      ⁢                                                                                          ⁢                                              ϕ                        0                                                                              )                                                      ]                                    sin          [                                          ⁢                      π            ⁢                                                  ⁢                                          d                λ                            ⁡                              (                                                      sin                    ⁢                                                                                  ⁢                    ϕ                                    -                                      sin                    ⁢                                                                                  ⁢                                          ϕ                      0                                                                      )                                              ]                            .  
As follows from the above expression for |G(θ)|, as the number or antennas N in the ULA increases, the power pattern becomes more and more narrow and increases the antenna gain at ϕ=ϕ0. Actually, as N→∞ the Dirichlet kernel approaches a Dirac delta function with infinitely small beam width and infinite antenna gain a ϕ=ϕ0. FIG. 1 illustrates the power pattern in terms of |G(θ)| as a function of angle θ for a DFT beam with N=10, ϕ0=5°.
At the same time, apart from creating (narrow) device-specific beams, the radio base station must be capable of also creating wide cell-specific beams to cover the entire sector/cell with a desired level of radiation. This might be needed, e.g., for sending broadcast information or common reference signals. Existing approaches to address this issue will be summarized next.
According to a first approach, a separate wide-beam antenna may be used for transmission of broadcast data. A drawback with this approach is that it requires additional hardware.
According to a second approach, broadcast data is transmitted using a single antenna array element, or sub-array, of the antenna. This array element or sub-array will have a wider beam than the full array of the antenna. A drawback of this approach is that only one, or a few, power amplifiers (PAs) in the antenna array is/are utilized, which thus wastes power resources.
According to a third approach, amplitude and/or phase tapering is used over the full array of the antenna to widen the beam. Drawbacks with such tapering are that amplitude tapering gives poor utilization of the PA resource and that it is in many cases not possible to synthesize the desired beam shape using phase-only tapering.
According to a fourth approach, broadcast data is transmitted sequentially in different directions using narrow beams. A potential drawback with this approach is that this takes longer time and consumes more resource elements than transmitting broadcast data simultaneously in all directions with a wide beam.
Other scenarios where it may be desirable to use wide beams with an antenna array with many elements is in millimeter-wave communications, which is an access technology foreseen to be a part of 5G radio access. Due to the increased propagation loss at such high frequencies, high gain beam forming may be needed to retain the link budget, possibly both at the receiver and transmitter. Beam forming may be needed since the dominant propagation paths between a transmitter and a receiver are typically not known a priori. Testing all combinations of a large number of narrow transmit and receive beams in order to find the best beam pair may consume a prohibitive amount of time/frequency resources. A way to resolve this issue may be for the radio base station to start the search procedure with wide beams and then make the beams narrower and narrower until the best pair of narrow beams has been found. Such a beam finding procedure generally requires means for generating beams with different beam widths in a flexible manner. In order to fully utilize the antenna array and the available PA resource it may be desired to use all antenna elements and all PAs at full power when transmitting beams with different beam widths.
Hence, there is a need for improved beam forming.